I have always been curious about how much business Steam gets. The release of FEAR 3 provides the opportunity for a rough estimate.

FEAR 3 has a steam achievement for reaching level 10.The global achievement stat percentage for this achievement is 5.8%.A quick skim through the ingame leaderboards shows that approximately 270 people have reached this point. There is some overlap between different boards, which I've somewhat adjusted for.270/0.058=4,655.

The #1 selling game on Steam has sold less than 5k copies/$250k. That's not a lot for a solid-B release. In fact, that's an upper bound. People that got their copy via retail or another digital distributor also count towards the 5k player estimate, as do preorders.

I'd take Steam's "Top sellers" list with a huge grain of salt. They may not list the games in order and even if they did, you don't know how they quantify "top seller". For all we know, it could be a list of top selling games and some other games that publishers paid to have included (this doesn't even have to go through Steam; publishers can just spend a few grand buying gift codes to manipulate the list, sort of like what happens on Apple's app store). Another thing to consider is that the achievement stats may not be in sync with the sales.